Engineering hierarchical hollow nickel sulfide spheres for high-performance sodium storage

Sodium-ion batteries (SIBs) are considered as promising alternatives to lithium-ion batteries (LIBs) for energy storage due to the abundance of sodium, especially for grid distribution systems. The practical implementation of SIBs, however, is severely hindered by their low energy density and poor cycling stability due to the poor electrochemical performance of the existing electrodes. Here, to achieve high-capacity and durable sodium storage with good rate capability, hierarchical hollow NiS spheres with porous shells composed of nanoparticles are designed and synthesized by tuning the reaction parameters. The formation mechanism of this unique structure is systematically investigated, which is clearly revealed to be Ostwald ripening mechanism on the basis of the time-dependent morphology evolution. The hierarchical hollow structure provides sufficient electrode/electrolyte contact, shortened Na+ diffusion pathways, and high strain-tolerance capability. The hollow NiS spheres deliver high reversible capacity (683.8 mAh g−1 at 0.1 A g−1), excellent rate capability (337.4 mAh g−1 at 5 A g−1), and good cycling stability (499.9 mAh g−1 with 73% retention after 50 cycles at 0.1 A g−1).